1. Technical Field
This invention relates to a light weight and compact fuel gas steam reformer design which is formed from a plurality of repeating gas flow modules. More particularly, this invention relates to a fuel gas steam reformer design which provides improved heat transfer between a burner gas stream and a fuel gas stream being reformed.
2. Background Art
Fuel cell power plants include fuel gas steam reformers which are operable to catalytically convert a fuel gas, such as natural gas, into hydrogen and carbon dioxide. The conversion involves passing a mixture of the fuel gas and steam through a catalytic bed which is heated to a reforming temperature of about 1,250xc2x0 F. to about 1,600xc2x0 F. Catalysts typically used are nickel catalysts which are deposited on alumina pellets. A typical reformer will consist of a plurality of reaction tubes which are contained in a housing that is insulated for heat retention. The reaction tubes are heated by burning excess fuel gas in the housing and passing the burner gases over the reaction tubes. The individual reaction tubes will typically include a central exhaust passage surrounded by an annular entry passage. The entry passage is filled with the catalyzed alumina pellets, and a fuel gas-steam manifold is operable to deliver the fuel gas-steam mixture to the bottom of each of the entry passages whereupon the fuel gas-steam mixture flows through the catalyst beds. The resultant heated hydrogen, carbon dioxide and carbon monoxide gas mixture then flows through the central exhaust passages in each tube so as to assist in heating the inner portions of each of the annular catalyst beds; and thence from the reformer for further processing and utilization.
Steam reformers require a large amount of surface area in the catalyst bed in order to provide a high degree of catalyst-fuel mixture interaction and a large heat transfer surface area to produce the amount of hydrogen required to operate the fuel cells at peak efficiency. This need for a large catalyst bed and heat transfer surface area, when met by using catalyst-coated pellets in tubular reformers, results in undesirably large and heavy reformer assemblies. For example, a commercially available 200 KW acid fuel cell power plant includes a steam reformer component which has a volume of about 150 to about 175 cubic feet; and weighs about 3,500 lbs.
It would be highly desirable to provide a light weight and compact steam reformer which provides enhanced heat transfer without the need of an excessively large heat-transfer area, and is suitable for use in a mobile fuel cell power plant.
This invention relates to a hydrocarbon fuel gas reformer design which provides enhanced heat transfer without the need to unduly increase the size of the reformer. The general construction of the reformer of this invention is somewhat similar to the reformer described in commonly owned U.S. Pant. No. 6,117,578, granted Sep. 12, 2000, in that both utilize catalyzed wall fuel and burner gas passages for compactness and light weight. The disclosure of the ""578 patent is incorporated herein in its entirety for purposes of enablement. The disclosure of commonly owned U.S. Pat. No. 5,733,347, granted May 31, 1998 is also incorporated herein in its entirety for purposes of enablement. The system described in the aforesaid ""578 patent suggests the use of a counter-flow design between the burner gas passages and the adjacent process gas passages, which results in the maximum heat transfer from the burner gases to the process gas stream at a point, i.e., its exit end, where minimal heat transfer is needed and minimal heat transfer from the burner gas stream to the process gas stream at a point, i.e., its entrance end, where maximum heat transfer would be more desirable. In this type of reformer, the entry burner gas temperatures are about 2,400xc2x0 F. and the exit burner gas temperatures are in the range of about 900xc2x0 F. to 1,000xc2x0 F.
Improved and more complete heat transfer is obtained by customizing the direction of flow of the fuel gas stream being reformed, and customizing the direction of flow of the burner gases which heat the fuel gas stream being reformed, in the improved heat exchange module. The reformer assembly modules will contain at least one process gas flow component which includes a sequence of process fuel gas flow paths; and at least one burner gas flow component which includes a sequence of burner gas flow passages. Each gas flow path component will include gas flow reversal manifolds which interconnect individual inlet gas flow passages and result in a reversal of the direction of gas flow to outlet gas flow passages in both the process fuel gas stream and in the burner gas stream so as to provide both co-flow and counter-flow of the process gas and burner gas streams.
Each of the process fuel gas components includes a pair of reforming process gas passages which are sandwiched around a regenerator process gas passage. The fuel gas stream to be reformed, which is referred to herein as the xe2x80x9cprocess gas streamxe2x80x9d, or the xe2x80x9cprocess gasxe2x80x9d, enters the fuel gas components through the two process gas passages flowing in the same direction, and then enters the regenerator process gas passage flowing in the opposite direction, thereby exiting the fuel gas components. A gas flow reversal manifold interconnects the reforming process gas passages with the regenerator process gas passage. The reformed process gas stream flowing through the regenerator process gas passage assists in heat transfer to the process fuel gas flowing through the adjacent reforming gas passages.
Each of the burner gas components includes at least two burner gas passages, which can be termed xe2x80x9cincomingxe2x80x9d and xe2x80x9coutgoingxe2x80x9d burner gas passages. The hotter burner gases flow into the burner gas component through the incoming burner gas passages and the lower temperature burner gases flow out of the burner gas component through the outgoing burner gas passages. Each of the incoming burner gas passages shares a common wall with one of the process gas passages in the reformer gas component so as to be disposed in heat exchange relationship with the one process gas passage; and each of the outgoing burner gas passages shares a common wall with the other of the reformer gas passages in the same reformer gas component so as to be in heat exchange relationship with the other of the reformer gas passages.
In each embodiment of the modules of the fuel gas reformer design of this invention, the high temperature burner gas stream flowing through the incoming burner gas passages in each of the burner gas components flows in the same direction as the first adjacent incoming process gas flow passage, a condition which we refer to herein as xe2x80x9cco-flowxe2x80x9d; and the lower temperature burner gas stream flowing through the outgoing burner gas passages flows in the opposite direction to the other incoming adjacent process gas flow passage in the reforming gas flow component, a condition which we refer to herein as xe2x80x9ccounter-flowxe2x80x9d. One of the objects of this invention is to maximize the amount of co-flow between the burner gas stream and the process gas stream, and minimize the amount of counter-flow, but not completely eliminate counter-flow. Thus, the incoming reformed gas stream in certain reformer gas stream components are subjected to heat exchange from both a co-flow burner gas stream, and heat exchange from a counter-flow burner gas stream, while others of the reformer gas stream components are subjected to heat exchange solely from co-flow burner gas streams. The co-flow, counter-flow design results in a more complete transfer of heat from the burner gases to the reformer or process gas without the need to unduly enlarge the reformer assembly, and also results in a lower burner gas stream outlet temperature. The incoming process gas stream is also subjected to heat exchange from the outgoing regenerated process gas stream. Each module may contain two process gas stream components combined with one burner gas stream component, as will be more fully explained hereinafter.
It is therefore an object of this invention to provide an improved heat exchange relationship between a burner gas stream component and a process fuel gas stream component in a hydrocarbon fuel gas reformer assembly which is suitable for use in a fuel cell power plant.
It is a further object of this invention to provide a hydrocarbon fuel gas reformer assembly of the character described which employs a combination of co-flow and counter-flow burner gas stream passages and process fuel gas passages in the burner gas and process gas components in each module.
It is yet another object of this invention to provide a hydrocarbon fuel gas reformer assembly of the character described which includes a counter-flow regenerator process gas passage which is disposed in heat exchange relationship with the reforming fuel gas passages in each of the process gas components.
It is a further object of this invention to provide a hydrocarbon fuel gas reformer assembly which is formed from repeating burner gas and process fuel gas passage modules.
It is an additional object of this invention to provide a hydrocarbon fuel gas reformer assembly which is able to utilize a single burner gas component to provide heat for reforming the hydrocarbon fuel gas in a plurality of process fuel gas components in a single module.
It is yet another object of this invention to provide a hydrocarbon fuel gas reformer assembly of the character described wherein the process gas components in a burner gas-process gas module will be subjected to a greater degree of co-flow with that burner gases than counter-flow with the burner gases.
These and other objects and advantages of this invention will become more readily apparent to one skilled in the art from the following detailed description of several embodiments of the invention when taken in conjunction with the accompanying drawings in which: